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Volume 14, issue 5 | Copyright
Ocean Sci., 14, 1185-1206, 2018
https://doi.org/10.5194/os-14-1185-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 09 Oct 2018

Research article | 09 Oct 2018

Turbulence and hypoxia contribute to dense biological scattering layers in a Patagonian fjord system

Iván Pérez-Santos1,2, Leonardo Castro2,3,4, Lauren Ross5, Edwin Niklitschek1, Nicolás Mayorga1, Luis Cubillos2,3, Mariano Gutierrez6, Eduardo Escalona2,3, Manuel Castillo2,7, Nicolás Alegría8, and Giovanni Daneri2,9 Iván Pérez-Santos et al.
  • 1Centro i-mar, Universidad de Los Lagos, Camino a Chinquihue km 6, Puerto Montt, Chile
  • 2Centro COPAS Sur-Austral, Universidad de Concepción, Campus Concepción, Víctor Lamas 1290, Casilla 160-C, código postal: 4070043, Concepción, Chile
  • 3Departamento de Oceanografía, Universidad de Concepción, Campus Concepción, Víctor Lamas 1290, Casilla 160-C, código postal: 4070043, Concepción, Chile
  • 4Centro de Investigaciones de Altas Latitudes (IDEAL), Universidad Austral de Chile, Valdivia, Chile
  • 5Department of Civil and Environmental Engineering, University of Maine, 5711 Boardman Hall, Orono, ME 04469-5711, USA
  • 6Universidad Nacional Federico Villareal, Facultad de Oceanografía, Pesquerías y Ciencias Alimentarias, Calle Francia 726, Miraflores, Lima, Peru
  • 7Centro de Observación Marino para Estudios de Riesgo en Ambientes Costeros, Facultad de Ciencias del Mar y de Recursos Naturales, Universidad de Valparaíso, Valparaiso, Chile
  • 8Instituto de Investigaciones Pesqueras, Talcahuano, Chile
  • 9Centro de Investigaciones en Ecosistemas de la Patagonia (CIEP), Coyhaique, Chile

Abstract. The aggregation of plankton species along fjords can be linked to physical properties and processes such as stratification, turbulence and oxygen concentration. The goal of this study is to determine how water column properties and turbulent mixing affect the horizontal and vertical distributions of macrozooplankton along the only northern Patagonian fjord known to date, where hypoxic conditions occur in the water column. Acoustic Doppler current profiler moorings, scientific echo-sounder transects and in situ plankton abundance measurements were used to study macrozooplankton assemblages and migration patterns along Puyuhuapi Fjord and Jacaf Channel in Chilean Patagonia. The dissipation of turbulent kinetic energy was quantified through vertical microstructure profiles collected throughout time in areas with high macrozooplankton concentrations. The acoustic records and in situ macrozooplankton data revealed diel vertical migrations (DVM) of siphonophores, chaetognaths and euphausiids. In particular, a dense biological backscattering layer was observed along Puyuhuapi Fjord between the surface and the top of the hypoxic boundary layer ( ∼ 100m), which limited the vertical distribution of most macrozooplankton and their DVM, generating a significant reduction of habitat. Aggregations of macrozooplankton and fishes were most abundant around a submarine sill in Jacaf Channel. In this location macrozooplankton were distributed throughout the water column (0 to  ∼ 200m), with no evidence of a hypoxic boundary due to the intense mixing near the sill. In particular, turbulence measurements taken near the sill indicated high dissipation rates of turbulent kinetic energy (ε ∼ 10−5Wkg−1) and vertical diapycnal eddy diffusivity (Kρ ∼ 10−3m2s−1). The elevated vertical mixing ensures that the water column is well oxygenated (3–6mLL−1, 60%–80% saturation), creating a suitable environment for macrozooplankton and fish aggregations. Turbulence induced by tidal flow over the sill apparently enhances the interchange of nutrients and oxygen concentrations with the surface layer, creating a productive environment for many marine species, where the prey–predator relationship might be favored.

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Fjord systems play an important role in primary production and carbon export. Acoustic, hydrographic and in situ abundance measurements were used to study macrozooplankton assemblages at 44.7° S. Diel vertical migration of zooplankton stopped at the hypoxic boundary layer and apparently did not tolerate the hypoxic conditions. Turbulence appears to be the oceanographic process that contributes to vertical mixing around the sill, helping the interchange of nutrients, feeding and carbon export.
Fjord systems play an important role in primary production and carbon export. Acoustic,...
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